Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semi-conducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1 s−1). In addition, it is important that the semi-conducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
A known compound which has been shown to be an effective p-type semiconductor for OFETs is pentacene [see S. F. Nelson, Y. Y. Lin, D. J. Gundlach and T. N. Jackson, Appl. Phys. Lett., 1998, 72,1854]. When deposited as a thin film by vacuum deposition, it was shown to have carrier mobilities in excess of 1 cm2 V−1 s−1 with very high current on/off ratios greater than 106. However, vacuum deposition is an expensive processing technique that is unsuitable for the fabrication of large-area films.
Regular poly(3-hexylthiophene) has been reported with charge carrier mobility between 1×10−5 and 4.5×10−2 cm2 V−1 s−1, but with a rather low current on/off ratio between 10 and 103 [see Z. Bao et al., Appl. Phys. Lett. 1997, 78, 2184]. In general, poly(3-alkylthiophenes) show improved solubility and are able to be solution processed to fabricate large area films. However, poly(3-alkylthiophenes) have relatively low ionisation potentials and are susceptible to doping in air [see H. Sirringhaus et al., Adv. Solid State Phys. 1999, 39,101].
It was an aim of the present invention to provide new organic materials for use as semiconductors or charge transport materials, which are easy to synthesise, have high charge mobility and good processability. The materials should be easily processable to form thin and large-area films for use in semiconductor devices. Other aims of the invention are immediately evident to those skilled in the art from the following description.
It has been reported in the literature that benzo[1,2-b:4,5-b′] dithiophene, hereinafter also shortly referred to as benzodithiophene with the following structure has a high charge carrier mobility and is useful as organic semiconductor. Benzodithiophene monomers or dimers, oligo- or polymers formed thereof and their use as an organic semiconductors have been described for example in Kossmehl et al., Makromol. Chem. 1983,184(3), 627-50, Katz et al., J. Mater. Chem. 1997, 7(3), 369-76, Laquindanum et al., Adv. Mater. 1997, 9(1), 36-39 and in U.S. Pat. No. 5,625,199.
In particular the dimeric bisbenzodithiophene has been shown to exhibit high charge carrier mobilities of 0.04 cm2V−1s−1. Its structure has flatter conformation than e.g. α-sexithiophene with comparable size. This enables compressed molecular packing and strong intermolecular interactions, which is favourable for compact stacking of the material and results in π—π-overlap and hence makes this compound an effective charge transport materials with high carrier mobilities. However, bisbenzodithiophene has a very high melting point over 400° C. and very low solubility in organic solvents, so that it cannot be readily solution processed and can only be vacuum deposited.
Therefore, another aim of the invention was to provide benzodithiophenes that are more easily processible in the manufacture of semiconductor devices.